The goal of these studies is to examine the influence of gonadal steroid hormones on the development of the nervous system, with the general aim of revealing principles underlying nervous system organization and development. Results of these studies to date indicate that neuronal growth in the hamster (Mesocricetus auratus) dorsomedial preoptic area (as measured by soma size changes and growth cones) reached a peak at 5 to 10 days of age and then declined. Measures of the size of the dendritic tree (from Golgi-stained material) showed a peak around 10 to 20 days of age followed by a net decrease. The decrease reflects the sum of two processes; degeneration of some dendrites and continuing growth of others. Sex differences in the frequency of growth cones were observed in the first 10 days of life. Adult-like sexually dimorphic dendritic patterns emerge by at least 30 days of age from these loss and growth processes. Sex differences in neuronal morphology were not observed in samples from the hippocampus or suprachiasmatic nucleus of 30 day or older animals.